1. Field of the Invention
The present invention relates to an apparatus for use in the preparation of gas and gas-vapor mixtures of accurately known composition, more particularly gas and gas-vapors mixtures of known composition for use in the calibration of gas chromatographs (GCs).
GCs are extensively used for the analysis of a variety of samples, the components of which may include gases, vapours and liquids. Frequently the analytes of interest in the sample are present in low, or even trace, concentrations. Since the raw data of gas chromatography are mostly obtained in the form of relative, rather than absolute, values GCs almost invariably require calibration with standard samples.
Calibration of GCs for liquid mixtures may be done by any one of several well known and satisfactory procedures.
Gas mixtures of accurately known (or at least specified) composition are readily available commercially for certain gas mixtures, but are expensive and seldom available as mixtures containing one or more uncommon components. In some cases, standard gas mixtures are known to deteriorate (i.e. change composition) with age. It would therefore be useful to have available an accurate and reliable apparatus and/or method with which to check the composition of commercially available standard gas mixtures.
2. Description of Related Art
Several devices and procedures have been described in the literature for the preparation of standard gas mixtures for use in calibrating GCs. These include gravimetric and volumetric methods, flow dilution systems, and methods utilising continuous vaporization into a gas stream, diffusion of a vapour through a capillary tube, and the use of a permeation tube. Some of these procedures and their attendant problems are reviewed in R. L. Grob (ed.) "Modern Practice of Gas Chromatography" (2nd Ed.) Wiley, 1985, and by Cowper and DeRose, "The Analysis of Gases by Chromatography", Pargamon, 1983. Most of the procedures described are subject to potential sources of error, in some cases serious sources of error, in particular because provision is seldom made for measurement of the temperature of the gases before mixing to determine, with a high degree of accuracy, the mass or the number of moles of each gas being mixed.
The most convenient and accurate method available for those wishing to prepare their own standards is the gravimetric method, i.e. by weighing. For accurate results the containers should be weighed in a vacuum, to eliminate buoyancy effects. This requirement presents severe practical difficulties in a most cumbersome procedure, particularly if a series of mixtures of different analyte concentrations is to be prepared, and effectively rules out this approach for all but the most sophisticated laboratories. Even in these laboratories, problems are encountered if the analyte is to be present in very low concentrations, e.g. in the parts per million range.
Of particular interest and relevance to the present invention are volumetric methods, such as described by the British Standards Institution BS 4559: Part 3: 1983 and in ISO standard 6144--1981: Part 3, "Static volumetric methods". Again, in the methods described in this document, no provision is made for temperature measurement of the gases or ensuring a uniform temperature before mixing. Also, reference to this document shows that the preparation of a series of standard gas mixtures of varying composition by any one of the two methods described is very cumbersome. Preparation of gas-vapour mixtures in which the vapour derives from a volatilisable liquid is given very little attention. For example, no provision is made for means to readily volatilise one or more liquid components to be introduced as the analyte(s) of interest.
A device called the Astro Digital Static Gas Blender marketed by Astro Int. Corp. of Texas and patented under U.S. Pat. No. 4,257,439 is known to applicant but is based on a sixty year old method by Burnett for studying PVT properties which is not relevant to the present invention.